1. Field of the Invention
The present invention relates to a liquid crystal display device. More particularly, the present invention relates to an in-plane switching mode liquid crystal display device which achieves efficient operations in both reflection and transmission modes while improving a contrast ratio thereof.
2. Discussion of the Related Art
A liquid crystal display device, a widely used flat panel display device, includes liquid crystal material having the fluidity of a liquid and the optical properties of a crystal. An electric field is applied to the liquid crystal to change the optical anisotropy of the liquid crystal. The power consumption of the liquid crystal display device is lower than that of a related art cathode ray tube, and the volume of the liquid crystal display device is less than that of the related art cathode ray tube. Furthermore, the liquid crystal display device can be manufactured in larger sizes and high definition. Consequently, the liquid crystal display device is widely used.
The liquid crystal display device further includes a color filter array substrate which forms an upper substrate and a thin film transistor (TFT) array substrate which forms a lower substrate. The upper and lower substrates are arranged to face each other, and a liquid crystal layer is formed between the upper and lower substrates, with the liquid crystal layer having dielectric anisotropy. The liquid crystal display device having the above described configuration is driven as TFTs added to hundreds of thousands of pixels are switched via address lines used for the selection of pixels, so as to apply a voltage to the corresponding pixels.
Based on the properties of a liquid crystal and the structure of a pattern, liquid crystal display devices may be constructed in various different modes.
Specifically, liquid crystal display devices may be classified into: a twisted nematic (TN) mode liquid crystal display device in which liquid crystal directors are arranged such that the liquid crystal directors are twisted 90°, and voltage is applied to the liquid crystal directors to control the liquid crystal directors; a multi-domain mode liquid crystal display device in which a pixel is divided into several domains, and directions of main viewing angles of the respective domains are changed to accomplish a wide viewing angle; an optically compensated birefringence (OCB) mode liquid crystal display device in which a compensating film is attached to a substrate to compensate for the change in the phase of light depending upon the path of the light, an in-plane switching mode liquid crystal display device in which two electrodes are formed on a substrate such that liquid crystal directors are twisted on the even plane of an orientation film, and the like.
Liquid crystal display devices may further be classified as a transmissive liquid crystal display device in which a backlight is used as a light source, a reflective liquid crystal display device in which external natural light is used as a light source instead of the backlight, and a reflective-transmissive liquid crystal display device for overcoming not only a problem of the transmissive liquid crystal display device in that the backlight has a requires an enormous power consumption, but also a problem of the reflective liquid crystal display device in that the use of the reflective liquid crystal display device is impossible when external natural light is dim.
The reflective-transmissive liquid crystal display device includes both a reflective region and a transmissive region in each unit pixel. Therefore, the reflective-transmissive liquid crystal display device can be used as the reflective liquid crystal display device and the transmissive liquid crystal display device as occasion demands.
In the transmissive region that is included in the transmissive liquid crystal display device and the reflective-transmissive liquid crystal display device, if light generated by the backlight is incident thereto by passing through the lower substrate, the transmissive region transmits the light to the liquid crystal layer, so as to achieve an increased brightness. The reflective region that is included in the reflective liquid crystal display device and the reflective-transmissive liquid crystal display device, if bright external natural light is incident thereto by passing through the upper substrate, the reflective region reflects the external light, so as to achieve an increased brightness.
To maximize the operational efficiency of each of the reflective region and the transmissive region, there has been proposed a dual-cell gap technology in which a cell gap in the transmissive region is approximately two times the cell gap in the reflective region.
A method for operating an in-plane switching mode liquid crystal display device in a transflective mode has been proposed. Even in this case, the operational efficiency of the liquid crystal display device in the transflective mode can be maximized by constructing electrodes on the basis of the dual-cell gap technology.
A related art in-plane switching mode liquid crystal display device, which is operated in a transflective mode, will be described with reference to the accompanying drawing.
FIG. 1 is a schematic view illustrating a related art in-plane switching mode liquid crystal display device.
As shown in FIG. 1, the related art in-plane switching mode liquid crystal display device includes a lower substrate 10 and an upper substrate 20 which are arranged to face each other, a liquid crystal layer 50 formed between the upper and lower substrates 20 and 10, a first polarizing plate 31 formed at an outer surface of the lower substrate 10, and a second polarizing plate 32 formed at an outer surface of the upper substrate 20. On the lower substrate 10 are defined a transmissive region 12 and a reflective region 11 separately.
The transmissive region 12 and the reflective region 11 constitute each pixel region where pixel electrodes (not shown) and common electrodes (not shown) are alternately formed. When a voltage is applied to the respective electrodes, the electrodes create a horizontal electric field to allow liquid crystals located between the pixel electrode and the common electrode to be oriented in a horizontal direction.
The lower substrate 10 is a thin film transistor array substrate that is formed with a plurality of lines and thin film transistors to apply a signal to the pixel electrodes and the common electrodes. The upper substrate 20 is a color filter array substrate that is formed with a color filter array.
Although not shown, on the lower substrate 10 are formed pluralities of gate lines and data lines so that the gate lines and the data lines cross with each other to define each pixel region. The thin film transistors are formed at respective crossings between the gate lines and the data lines. A gate insulation film is formed as an interlayer film between the gate lines and the data lines, and a protective film is formed as an interlayer film between the data lines and the pixel electrodes.
In the above described structure, a portion of the liquid crystal layer 50, which corresponds to the reflective region 11, has an optical path two times of that of the remaining portion of the liquid crystal layer 50 that corresponds to the transmissive region 12. Therefore, a cell gap in the reflective region 11 may be determined to be a half a cell gap in the transmissive region 12. In this case, the regulation of the cell gap is accomplished by regulating thicknesses of the gate insulation film and the protective film formed in each of the reflective region 11 and the transmissive region 12.
The gate insulation film and the protective film in the reflective region 11 are removed by a predetermined thickness. To maximize the operational efficiency of the liquid crystal display device in a transmission mode, the reflective region 11 and the transmissive region 12 are provided with a dual cell gap to match On/Off modes of the reflective region 11 and the transmissive region 12. A ratio of the cell gap (d1) in the transmissive region to the cell gap (d2) in the reflective region may be approximately 2:1.
Accordingly, light incident to the reflective region and light incident to the transmissive region simultaneously reach a screen surface where an image is displayed. In other words, if external natural light is incident to the reflective region from the upper side, the light reaches the screen surface after reciprocally passing through the liquid crystal layer 50. Also, if light from the backlight is incident to the transmissive region, the light reaches the screen surface after passing through the liquid crystal layer in the transmissive region that has a cell gap two times that of the reflective region. As such, the external natural light and the light from the backlight simultaneously reach the screen surface.
On an inner surface of the lower substrate 10 and the upper substrate 20 are formed first and second orientation films (not shown), respectively, to allow liquid crystal molecules of the liquid crystal layer 50 to be oriented in a predetermined direction. The first and second polarizing plates 31 and 32 are provided on the outer surfaces of the lower substrate 10 and the upper substrate 20, respectively. Between the upper substrate 20 and the second polarizing plate 32 may be further provided a phase difference plate (not shown) that serves to retard a phase difference.
The first polarizing plate 31 and the second polarizing plate 32 function to pass only light incident thereto in a direction parallel to a light transmission axis, so as to convert natural light into linearly polarized light. The phase difference plate functions to change the polarized state of light by retarding the phase of the linearly polarized light incident thereto by an angle of 180°. In the related art, a half wave plate (HWP), which has a phase difference corresponding to λ/2, is used as the phase difference plate.
By regulating a transmission axis of any one of the polarizing plates 31 and 32 and a transmission axis of the phase difference plate, and an angle of directors of liquid crystal molecules, the liquid crystal display device may have a normal black mode.
Specifically, an optical axis of the phase difference plate, i.e. the half wave plate, is aligned at an angle of +θ from a transmission axis of the upper polarizing plate and in turn, a transmission axis of the lower polarizing plate is aligned at an angle of +θ from the optical axis of the half wave plate. Also, the liquid crystals are initially oriented in a direction of +45° from the transmission axis of the lower polarizing plate. In this case, if the liquid crystals are driven, the polarization direction of light to be emitted is rotated by an angle of −45° toward the transmission axis of the lower polarizing plate, to thereby realize a white level.
Considering first the reflective region, when the liquid crystals are not driven (i.e. in an Off state), the polarization direction of external natural light incident to the upper polarizing plate is rotated by an angle of 2θ by passing through the phase difference plate. Subsequently, the light is changed into circularly polarized light while passing through the liquid crystals and then, reaches a reflective plate. If the circularly polarized light is reflected by the reflective plate, the circularly polarized light is again changed into the linearly polarized light while passing through the liquid crystals. Thereafter, the polarization direction of the linearly polarized light is rotated by an angle of 2θ by passing through the phase difference plate. As a result, the light to be emitted has an angle of 90° with the transmission axis of the upper polarizing plate. However, the light cannot pass through the transmission axis of the upper polarizing plate, resulting in a black level.
If a liquid crystal cell gap in the reflective region is d/2 (i.e. Δnd) and a cell gap for the liquid crystal layer is d/2, the liquid crystals serve as a quarter wave plate (QWP) having a phase difference corresponding to λ/4, thereby changing the linearly polarized light into the circularly polarized light and the circularly polarized light into the linearly polarized light.
Also, when the liquid crystals are driven (i.e. in an ON state), the polarization direction of the external natural light incident to the upper polarizing plate is rotated by an angle of 2θ by passing through the phase difference plate. The light reaches the reflective plate after passing through the liquid crystals without any change. If the light is reflected by the reflective plate, the light again passes through the liquid crystals without any change. Thereafter, the polarization direction of the light is rotated by an angle of 2θ by passing through the phase difference plate. As a result, the light to be emitted has the same direction as the transmission axis of the upper polarizing plate. Therefore, the light finally passes through the upper polarizing plate, resulting in a white level. In the case where the liquid crystals are driven, the liquid crystals are rotated by an angle of −45° to thereby be oriented in the same direction as the transmission axis of the lower polarizing plate.
Meanwhile, considering the transmissive region, when the liquid crystals are not driven (i.e. in an Off state), the polarization direction of light incident from the backlight to the lower polarizing plate is changed by an angle of 90° by the liquid crystals that are oriented in their initial direction, and is further changed by an angle of 2θ by the phase difference plate. As a result, the light to be emitted has an angle of 90° with the transmission axis of the upper polarizing plate. Accordingly, the light cannot pass through the upper polarizing plate, resulting in a black level.
In this case, if a liquid crystal cell gap in the transmissive region is d (i.e. 2Δnd) and a cell gap for the liquid crystal layer is d, the liquid crystals serve as a half wave plate (HWP) having a phase difference corresponding to λ/2, thereby changing the polarization direction of the light. That is, the polarization direction of the light is symmetrically changed on the basis of the orientation direction of the liquid crystals.
Also, when the liquid crystals are driven (i.e. in an ON state), the light incident from the backlight to the lower polarizing plate passes through the liquid crystals without any change and then, the polarization direction of the light is changed by the phase difference plate. As a result, the light to be emitted has the same direction as the transmission axis of the upper polarizing plate, resulting in a white level. In the case where the liquid crystal cells are driven, the liquid crystals are rotated by an angle of −45°, to thereby be oriented in the same direction as the transmission axis of the lower polarizing plate.
However, differently from that of a transmissive in-plane switching mode liquid crystal display device, the polarization state of light is changed via dual refraction obtained by liquid crystals (serving as a HWP) and the phase difference plate, such as a HWP, in the transmissive region of the reflective-transmissive in-plane switching mode liquid crystal display device. Therefore, there is the risk that light having an unintentional polarization direction, such as an elliptically polarized light, etc., may be generated, resulting in a little brightness at a black level. This problem deteriorates the superiority of the black level as one of essential characteristics of IPS.